Crystal-plastic deformation in seismically active carbonate fault rocks

Abstract

Seismic slip and aseismic creep commonly occur in distinct portions of the lithosphere due to the different dependencies of the underlying deformation mechanisms on conditions such as pressure and temperature (Scholz, 1998). Frictional failure involves dilatant processes facilitated by low confining pressures at shallow depths (Sammis & Ben-Zion, 2008;Sammis et al., 1987), whereas viscous deformation occurs by thermally activated processes promoted by higher temperatures at greater depths (Bürgmann & Dresen, 2008;Sibson, 1982). However, the temperature increase through shear heating during seismic faulting (Rice, 2006) challenges this strict separation by potentially activating temperature-dependent deformation mechanisms, such as crystal plasticity and diffusion creep (Nielsen, 2017). Depending on the material, melting, or decomposition reactions can also occur at high temperatures, leading to severe microphysical changes that severely alter the mechanical behavior of faults (Di Toro et al., 2011;Niemeijer et al., 2012). The main factor limiting the operation of crystal plasticity in the brittle regime is the extremely short duration of the temperature increase during and after fault slip. Thermal models predict a temperature drop through thermal diffusion within one second after sliding ceases to a value similar to the background temperature (Demurtas et al., 2019;Di Toro & Pennacchioni, 2004). Therefore, a key objective of earthquake geology is to assess the extent to which thermally activated processes impact fault structure and properties e.g., modifying the microstructure or activation of deformation mechanisms, during the short interval of coseismic slip.Deformed carbonates from principal slip zones of natural and experimental faults commonly exhibit crystallographic preferred orientations (CPOs) (